Litre

• 1. Introduction

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  The litre once carried the name "Cadil", and its earliest standards still sit on display at the Musée des Arts et Métiers in Paris. Introduced in France in 1795 as one of the new republican units of measurement, it was defined simply as one cubic decimetre. That makes it a cube measuring ten centimetres on every side, ten by ten by ten. Yet for something so plainly geometric, the litre has lived a surprisingly unsettled life. Its definition has been changed, reverted, and changed again. Its very symbol has split into two competing forms. And its famous link to the kilogram, the neat idea that a litre of water weighs almost exactly one kilogram, turns out to be only almost true. Why did scientists keep redefining a humble cube? Why does a single unit need two symbols? And how did a Byzantine measure of weight end up naming a measure of volume? The story of this everyday unit runs deeper than the milk carton suggests.

• 2. The Water And The Kilogram

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  One litre of liquid water has a mass of almost exactly one kilogram, and that is no accident. The gram was originally defined in 1795 as the mass of one cubic centimetre of water at the temperature of melting ice. The kilogram followed the same logic, defined as the mass of one cubic decimetre of water. So the unit of volume and the unit of mass were deliberately tied together through water. The word "almost" matters here. The original gram definition was abandoned in 1799, because the density of water changes with temperature and, very slightly, with pressure. The density also depends on the isotopic ratios of the oxygen and hydrogen atoms in a given sample. Modern measurements of Vienna Standard Mean Ocean Water, pure distilled water with an isotopic makeup representing the average of the world's oceans, give a density of 0.999975 at its point of maximum density. That maximum density occurs at 3.984 degrees Celsius, under one standard atmosphere of 101.325 kilopascals. Because the metre and kilogram were later redefined, the tidy one-to-one relationship is no longer exact. Still, the rule of thumb endures. A millilitre of water has a mass of about one gram, and a thousand litres of water has a mass of about a thousand kilograms, which is one tonne.

• 3. A Definition That Would Not Settle

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  From 1901 to 1964, the litre was not a cube at all. At the 3rd CGPM conference in 1901, it was redefined as the volume occupied by one kilogram of pure water at its temperature of maximum density, 3.98 degrees Celsius, under one atmosphere of pressure. The kilogram itself was specified as the mass of the International Prototype of the Kilogram, a particular platinum and iridium cylinder. The trouble was that the cylinder turned out to be about 28 parts per million too large. As a result, throughout that period a litre was actually about 1.000028 cubic decimetres, a tiny discrepancy with a precise cause. In 1964, at the 12th CGPM conference, the older definition was restored. The litre once again became simply another name for the cubic decimetre, exactly one dm3, severed from any dependence on a metal cylinder. The unit had wandered between geometry and mass and back to geometry. Earlier, in 1879, the CIPM had adopted the litre with the symbol lowercase l. The litre is not itself an SI unit; the SI unit of volume is the cubic metre. But the CGPM, the body that defines the SI, accepts the litre for use alongside it and sets out its acceptable symbols.

• 4. One Unit, Two Letters

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  Originally the only symbol for the litre was lowercase l, the letter L written small. This followed the SI convention that only unit symbols abbreviating a person's name begin with a capital letter. The problem was handwriting. In many English-speaking countries the digit 1 is written as a plain vertical stroke, with no upstroke, making it almost identical to a lowercase l. In some computer typefaces the two are barely distinguishable. To resolve the confusion, the CGPM adopted uppercase L as an alternative symbol in 1979, at its 16th conference. It also expressed a preference that only one of the two symbols should eventually survive. But in 1990 the International Committee for Weights and Measures said it was still too early to choose. The two forms split along regional lines. The United States National Institute of Standards and Technology recommends uppercase L, a practice followed widely in Canada and Australia, where prefixed forms appear as mL and microL. In the United Kingdom, Ireland, and the rest of Europe, lowercase l is used with prefixes, so a wine bottle reads 750 ml while a juice carton often says 1 litre. A third form once circulated too. A script l was recommended by the South African Bureau of Standards publication M33 and by Canada in the 1970s, and its use remains common in Japan and South Korea.

• 5. From Litron To Litre

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  The name "litre" did not begin as a measure of volume. It descends from an older French unit, the litron, which equalled roughly 0.82 litres. That name in turn came from Byzantine Greek, where it denoted a unit of weight rather than volume, reaching French through Late Medieval Latin. So a word for weight became a word for capacity. The litre served as a base unit in the original French metric system and carried through several later versions. Spelling, like the symbol, divides the world. The International Bureau of Weights and Measures uses "litre", the form shared by most English-speaking countries, while "liter" predominates in American English. The unit even has an obsolete twin. A litre is equal in volume to the millistere, a non-SI metric unit once customarily used for dry measure. The history hides small numerical drift, too. The original decimetre length was 44.344 lignes, revised in 1798 to 44.3296 lignes, which made the very first litre about 1.000974 of today's cubic decimetre. It was against that early litre that the kilogram was constructed.

• 6. Prefixes And Daily Life

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  Though not an official SI unit, the litre takes SI prefixes freely, and one stands out: the millilitre, one-thousandth of a litre, also called the cubic centimetre. It is everywhere in medicine, cooking, and automotive engineering. Some bodies push back on the less standard prefixes. In the United States, NIST advises using the millilitre or litre rather than the centilitre. Outside the powers of a thousand, Europe leans on prefixes that elsewhere fall out of favour. The hectolitre is the typical unit for production and export volumes of milk, beer, soft drinks, and wine in many European countries, and for fishing catches and quotas. Decilitres appear in cookbooks and on menus in Croatia, Switzerland, and Scandinavia. Centilitres mark the capacity of drinking glasses and small bottles. In colloquial Dutch in Belgium, a "vijfentwintiger" and a "drieendertiger", literally a twenty-fiver and a thirty-threer, name the common beer glasses, whose bottles read 25 cL and 33 cL. Bottles also come as 75 cL, half-size 37.5 cL for artisanal brews, or 70 cL for wines and spirits; cans arrive in 25, 33, and 50 cL; and shots on menus are often marked at about 3 cL. Countries that adopted the metric system after the SI standard tend to skip non-thousand prefixes entirely. In Canada, Australia, and New Zealand, consumer beverages are labelled almost exclusively in litres and millilitres. One exception is pathology, where blood lead and blood sugar levels may be quoted per decilitre.

• 7. Spoken And Scaled

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  In spoken English, "mL" for millilitre can be pronounced "mil", which invites confusion with several unrelated words. There is "mm" for millimetre, a thousandth of a metre. There is "mil" for a thousandth of an inch. There is the Scandinavian "mil", equal to ten kilometres, and "mil" again as a unit of angular measurement. Older abbreviations linger in technical corners. The abbreviation "cc", for cubic centimetre and equal to a millilitre, belongs to the cgs system that preceded the MKS system, which later became the SI. "cc" is still common in medical dosage and in describing combustion engine displacement. The microlitre was once called the lambda, written with the Greek letter, though that usage is discouraged; in medicine it is sometimes written mcL on test results. At the far ends of the scale, the litre stretches enormously. The Northern Territory Government has used kilolitres, megalitres, and gigalitres to measure water consumption, reservoir capacities, and river flows, even as cubic metres serve alongside them. Cubic metres are generally reserved for non-liquid commodities such as sand and gravel, or for storage space. The same prefix logic runs both ways, down to the quectolitre at ten to the minus thirtieth and up to the quettalitre at ten to the thirtieth, a single cube of water scaled across the whole measurable universe.